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A bandpass filter is an optical thin film that allows only a specific range of wavelengths to pass through while blocking all others. It is commonly used to improve sensing accuracy or enhance image contrast.
Based on design, bandpass filters can be categorized into narrowband and broadband types: narrowband filters allow only a very limited spectral range to pass through, making them suitable for high-precision detection; broadband filters cover a wider range and are often used for general imaging or color separation purposes.
The mainstream design is a multilayer interference structure, formed by stacking materials with high and low refractive indices (such as SiO₂ and TiO₂). These are produced using evaporation or sputtering processes to precisely control the passband position and cutoff performance.
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